The present invention relates to a substrate for mounting an electronic part (e.g. an electronic device), in which a solder film is formed on metallization layers, and an electronic part, in which a solder film is formed on a connection point or a lead surface.
Solder is formed on a substrate, an electrode of an electronic part or a lead of an electronic part, in very many products. Specifically, the following measures are frequently used in mounting of an electronic part: (1) forming metallization layers, which is to serve as an electrode, on a substrate, forming a solder film on the metallization layers, and using the solder film for connection to the electronic part; (2) forming metallization layers on an electrode of an electronic part, forming a solder film on the metallization layers, and using the solder film for connection to another electronic part; and (3) forming a solder film on a lead surface of an electronic part, which includes a lead, and also melting the solder film to carry out connection at the time of connection to a printed circuit board.
Metallization layers of an electrode and a solder film are formed exemplarily by: (1) coating a solder film on a copper foil on a printed circuit board with the plating method; and (2) forming metallization layers on a ceramic substrate and forming a thin solder film on the metallization layers by means of sputtering or vapor deposition. An electronic part formed with metallization layers of an electrode and a solder film is structured exemplarily such that a circuit element is formed on a semiconductor wafer and solder bumps are formed on electrode metallization layers of a connecting portion thereof. An electronic part, in which a plated layer consisting of Sn or a plated layer of a Sn alloy is formed on a lead surface of the electronic part, is an example, in which a solder film is formed on a lead surface of an electronic part.
A role which a solder film plays on a substrate or an electronic part will be described below.
In case of beforehand forming a solder film on a substrate, an electronic part is mounted so that a connecting portion of the electronic part abuts against the solder film, the semi-product is subjected to reflow to cause melting of the solder film provided on the substrate, and the solder wets and spreads over metallization layers provided at a connecting portion of the electronic part, etc., to achieve connection of the substrate and the electronic part.
In a sub-mount part, in which metallization layers is formed on a substrate containing ceramic, Si, etc. and a solder film is formed on the metallization layers by means of the thin film forming technique, an electronic part such as an optical element is pressed against the solder film, heating in a state without flux is effected to melt a thin film solder to cause the solder to wet and spread over the metallization layers of the electronic part for connection.
In a part, in which solder bumps are formed on an electrode metallization layers of an electronic part, solder bumps are in many cases formed by melting solder balls and causing the solder to wet and spread over the metallization layers of the electronic part when after being divided by dicing, solder bumps are to be formed before separating into chips. In this case, connection is achieved by mounting the electronic part on a printed circuit board, a ceramic substrate, etc. and make it being subjected to reflow, so as to melt the solder bumps and cause the solder to wet and spread over the metallization layers of the substrate. Also, in recent years, a circuit element is in some cases formed on a Si wafer and a method such as plating is used to form a solder film at the wafer level in a process prior to division by dicing.
For an electronic part including leads, solder paste is printed on a substrate, the leads of the electronic part are mounted on the paste, and the whole is subjected to reflow so as to melt the solder to connect the substrate and the leads of the electronic part to each other.
A plated film consisting of Ag or a plated film consisting of Sn is frequently used for lead frames of an electronic part. The plated film consisting of Ag is not oxidized at its surface and so excellent in wettability. While the plated film consisting of Sn is oxidized at its surface but a part of the oxidized film become broken somewhere and the solder on a side of a substrate and the Sn plated film melt to unite with each other so that connection is achieved.
Japanese Patent Laid-open No. Hei 5-190973 discloses a sub-mount for semiconductor laser, which is an example of the substrate for mounting electronic part. The sub-mount adopts Ti/Pt/Au as metallization layers and provides a Pt layer and an Au—Sn solder layer in a region, in which a semiconductor laser is to be mounted. Metallization layers are also formed on a back surface of the semiconductor laser and the semiconductor laser is fixed firmly by melting the Au—Sn solder on the sub-mount to connect it to the metallization layers.
The reason why an Au—Sn solder has been used in this technical field is that the Au—Sn solder is high in hardness and creep deformation is hard to occur. This is because when the semiconductor laser generates heat at the time of light emission to be raised in temperature to cause the solder to undergo creep deformation, the semiconductor laser is shifted in position and so optical coupling cannot be obtained.
In recent years, a semiconductor laser, etc. by a GaAs semiconductor has been frequently used as a light source for optical recording. Such semiconductor laser is affected by residual stress due to solder connection when the Au—Sn solder is used, resulting in decreased reliability in some cases. Such residual stress is generated since there is a difference in coefficient of thermal expansion between a semiconductor laser and a sub-mount when the semiconductor laser and the sub-mount are fixed at the fusing point of the solder and cooled to around room temperature. In the case where the solder is soft, the solder is deformed to relax the residual stress. However, in the case where the solder is hard, the effect of relaxing the residual stress is small.
Accordingly, if the Au—Sn solder is used to achieve connection of a semiconductor laser, of which an element is large in total length, a relatively large residual stress is generated in the semiconductor laser to shorten the life of the semiconductor laser in some cases.
In view of such a background, the use of a solder containing Sn, which is soft, as a main component for mounting of the semiconductor laser has come under consideration.
However, a solder film containing Sn as a main component and formed on a substrate, metallization layers of an electronic part, or a lead surface of an electronic part is, in many cases, formed at a surface thereof with an oxide film. This is because a solder ordinarily contains Sn as a main component and Sn is oxidized in the atmosphere.
With a view to surely achieving connection, it is convenient to use flux to reduce an oxide film present on a surface of the solder film to improve the connection property sharply. However, in recent years, it is often not permissible to use flux.
For example, failure in mounting an optical element is caused when flux residues are present in a light emitting portion of the optical element to intercept an optical path. Also, there may be damage to an optical element caused by flux itself or an organic solvent used for cleaning of flux residues.
In case of forming a solder film on a printed circuit board or a ceramic substrate, in case of forming solder bumps on a side of an electronic part, and in case of forming a solder film on a lead surface of an electronic part, respectively, flux has been hitherto used to decrease an adverse influence caused by the oxide film made on a surface of the solder film. In recent years, however, it has been increasingly promoted to make a connecting portion minute and to make a pitch small, and thus vaporization of a flux component and flow of flux at the time of connection may cause positional shift of a minute connecting portion to generate a short-circuit failure. Also, the material cost, coating process, and a subsequent cleaning process, respectively, of flux itself inherently constitute factors for an increase in cost, and so it is preferable to enable achieving connection simply in a fluxless manner.
It is estimated that a connecting portion is increasingly made minute for lead frames of electronic parts in the future, and solder paste printing on a substrate is approaching a limit in minuteness. That is, if connection can be achieved using solder plating on a side of an electronic part or the like without applying solder paste printing on a substrate, it is possible to sharply decrease failure in bridge between leads. Also, the use of flux possibly shifts a position of an electronic part, although slightly, due to generation of bubbles of flux at the time of heating or the like. Conventionally, since an amount of solder used was much, self-alignment caused by surface tension of molten solder prevented the positional shift of the electronic part, which is possibly caused by bubbles of flux. However, it can be expected that influences of such bubbles cannot be neglected when the amount of solder used is decreased as minuteness is promoted in the future. Also, since the use of flux is inherently a cause for an increase in cost in terms of material cost, flux coating process, and a subsequent cleaning process, a connection process, in which flux in as small in amount as possible is used, is desirable.
Also, a lead frame plated with Sn always involves a problem that a needle crystal called whisker grows to cause a short-circuit between leads. Making a lead pitch minute means that a short-circuit may be caused by even a shorter whisker, and thus, a further strict control will be demanded.